Skip navigation
CLN bookstore
× You have 2 more free articles available this month. Subscribe today.

Review of Prior Research Identifies Three Main Sources of Bias in Jury Decision-Making Processes

by Casey J. Bastian

In our adversarial court systems, a person impaneled on a jury is tasked with a seemingly straight-forward duty: determine the reliability and credibility of trial evidence, deliberate, and render a decision. However, studies reveal that this process is not so simple. Jurors are often irrational — frequently reaching inaccurate decisions when biased by various factors. Researchers set out to reveal these biases and to what extent they can undermine the proper functioning of the trial system.

As these are important questions, researchers Lee J. Curley and James Munro of Open University in England, along with Itiel E. Dror of University College London, initiated a review of dozens of studies previously published on sources of juror bias and its impact. Their findings were compiled in a report entitled Cognitive and Human factors in Legal Layperson Decision Making: Sources of bias in juror decision making. (“Report”). Researchers intended to “explore the potential sources from which biases are introduced into the jury.” Using findings about the impact biases have on juror decision-making, the Report also highlighted specific recommendations that may reduce the potential for the introduction of bias. The Report goes on to suggest that further areas of research into reducing the influence of these biases is warranted.

The Report evaluates prior research focused on the three main stages of trial. These stages are: “1) pre-evidence presentation (pre-trial publicity); 2) during evidence presentation (cognitive bias and bias from experts); and 3) post-evidence presentation (i.e., during deliberations).” In the Report, bias is defined as “a factor that produces a preference towards a certain outcome (acquittal or a conviction).” It is clear that injustice can result from bias and unfair decision-making by jurors. In one example, Sajid Qureshi was incarcerated for four years after being convicted by a panel of jurors of which several admitted they “had made up their minds before hearing the evidence.” 

The Report notes that the focus of the review was on materials concerning jury systems in England, Wales, Northern Ireland, and Scotland. While the focus was on these countries’ systems, some of the research was likely conducted in other countries, including the United States. That’s the reason why only “universal factors that are likely to influence jurors in the jurisdictions mentioned above” were closely examined by the Report’s authors.

Defendants in any justice system are entitled to a fair trial. Any influence from bias undermines this process. The Report identified three main points of bias towards which special attention should be given in efforts to ensure justice. Within these trial stages, the sources of bias are: 1) pre-trial bias arising from previously held beliefs and attitudes; 2) cognitive biases; and 3) bias from external legal actors, i.e., expert witnesses, who typically influence interpretations of presented evidence.

While research in recent decades has more specifically identified biases, as well as their causes and effects, general bias is an inherent phenomenon in any jury system. This reality has been a consideration of judicial administrators from the earliest times. This is one reason for the multi-person jury system. The intent is to have any individual biases “averaged out.” In this way, biases can be equally spread out across the jury during deliberations. An example given is one of bias resulting from race. Imagine there is a bias present in the jury that Black people are more likely to commit crimes than whites. To “average out” such a bias, there would need to be an equally prevalent counter-belief that whites are more likely to commit crimes.

This perfect balance of assumptions and conditions in a jury panel is unlikely. So, the basis of a need for the multi-person jury is well-intentioned but does little to actually alleviate the pernicious effects of bias that are inherently present. This “averaging out” just doesn’t typically occur naturally. The need to further understand bias and how to diminish any negative impacts becomes increasingly imperative as this information becomes more known. Pre-trial biases and attitudes, including personality and political persuasion, are among a number of individual differences that can impact a decision made by a juror. The Report discussed a review of literature focusing on specific personality traits and/or political persuasions rather than the typical focus on effects of global attitudes.

The Juror Bias Scale (“JBS”) is an early attempt to measure the influence of pre-trial biases on verdict choices. The JBS is a 17-item questionnaire consisting of two constructs: 1) the probability of commission construct and 2) reasonable doubt construct. The probability of commission construct reflects the extent to which prior beliefs and attitudes influence a belief that the accused committed the crimes charged. The construct highlights the level of guilt the juror perceives of the defendant. There are nine items found to adequately gauge the probability of commission construct. An example of a construct-related question used in the JBS is: “defense lawyers don’t really care about guilt or innocence; they are just in business to make money.” The probability of commission construct reveals attitudes related to conviction bias — the higher the score, the more likely a juror has a pro-prosecution bias.

The reasonable doubt construct is measured through eight items that reveal “how certain the juror needs to be before convicting.” An example of one item in this construct is: “For serious crimes like murder, a defendant should be found guilty so long as there is a 90% chance that he committed the crime.” JBS scores in both constructs range from 17 to 85, and in each, the defense would benefit from a lower score. The JBS reveals that the “degree of bias shown by the juror has a notable influence on the verdict they give.” Since the development of the JBS, subsequent research has measured the effects of numerous pre-trial biases and their impact on juror decision-making. An additional measurement device was later developed, and it is known as the Pre-trial Juror Attitude Questionnaire (“JPAQ”).

JPAQ consists of six separate constructs or biases. These are: 1) conviction proneness; 2) system confidence; 3) cynicism towards the defense; 4) social justice; 5) racial bias; and 6) innate criminality. All types of bias are not equal; some of the constructs can have a greater impact on juror outcomes. Conviction proneness has a stronger relationship with pre-deliberation verdicts than constructs such as system confidence, social justice, or innate criminality. And constructs such as cynicism towards the defense and racial bias had no significant association with pre-trial deliberation outcomes.

Some previous researchers have combined analysis of the JBS, TPAQ, and the 23-item Revised Legal Attitudes Questionnaire (a measure of legal authoritarianism) (“RLAQ”) to ascertain juror interpretations of the “beyond a reasonable doubt” standard. Typically, juror interpretations of “beyond a reasonable doubt” are highly indicative of likelihood of conviction. Jurors who hold a lower standard of proof are more likely to convict a defendant.

Pre-trial bias can also enter the courtroom through pre-trial publicity. In the past, only high-profile cases or celebrity trials were influenced by publicity. But today, in the digital age, biasing information concerning the accused is often shared on the internet and social media sites. Or, as the Report identifies it, a “type of viral bias.” Pre-trial publicity has been consistently shown to bias juror determinations.

The Report also notes that while the individual effects of pre-trial bias may be small, these biases have a tendency to “snowball” as they interact with cognitive biases and biases from expert witnesses during a trial. Cognitive bias is a description of subjective perceptions of the individual. One’s perceptions influence the decisions we make and how we interact with the world around us. Cognitive bias is primarily the result of two issues: “1) Homo Sapiens having a limited cognitive capacity and thus striving for efficiency when making decisions; and 2) personal and subjectively perceived experiences gained from the environment.” While cognitive shortcuts can be beneficial, this process, and associated resulting biases, can often lead to erroneous judgments. Neither experts in any particular field nor laypersons (and such non-experts in the legal field typically comprise a jury panel) are exempt from the effects of cognitive biases.

Pre-decisional distortions are another form of cognitive bias. This distortion is defined as: “jurors’ biased interpretation of new evidence to support whichever verdict is tentatively favored as a trial progresses.” Meaning jurors can tend to favor a verdict, and this tendency then distorts subsequently presented evidence; they begin to perceive evidence in a way that supports the verdict currently leading. This concept is similar to confirmation bias.

Ambiguous evidence can exaggerate the effects of pre-decisional distortion and confirmation bias – and jury trials are inherently ambiguous. Strong evidence leads to confessions, and weak evidence never reaches the courtroom. What is left is two opposing sides, competing to convince jurors that their “version of events” is closest to the truth. This becomes a “perfect environment for cognitive bias to thrive.”

In ambiguous environments, laymen can be overwhelmed by terminology or confused due to the nature of the adversarial system itself. This is described by the “elaboration likelihood model.” When an environment is not “cognitively taxing,” individuals will use cognitive resources to render a decision based mostly on evidence, referred to as a “high elaboration likelihood.” When there is a lack of knowledge or interest regarding the consequences, this leads to a “low elaboration likelihood.” In such circumstances, decision-makers are much more likely to reserve cognition and instead rely on previous biases. Cognitive processes and pre-trial attitudes also interact with the characteristics of both defendant and victim, further amplifying the effects of bias in the courtroom. Frugal cognitive processes combined with previous beliefs or attitudes and stereotypes “can lead to biased and erroneous decision making.”

Expert testimony is intended to assist a jury’s understanding of presented evidence. This requires that jurors evaluate the evidence testing processes to make a more informed decision. Expert testimony has a “major influence on juror perception of evidence strength,” but the influence isn’t always positive. The trial process would be better served if jurors didn’t simply assume the veracity of the expert testimony. Expert witnesses are not always rational and impartial decision-makers themselves. In addition to this inherent issue, the influence of their testimony is impacted by multiple additional factors. This includes: the complexity and type of evidence for which they are testifying; whether the expertise is actuarial or clinical; the appearance, attractiveness, or gender of the expert; pay rate and frequency of testimony; manner of presentation; and their willingness to testify about their own doubts and biases.

The use of task-irrelevant contextual information by expert witnesses can also cause jury-level issues, snowballing the effects of biasing in the adversarial process. As experts can also experience bias, their misperceptions of evidence serve as an additional channel that introduces bias. Frequently working closely with either the defense or the prosecution (whichever side employs them) can result in non-neutral testimony that is influenced by their employment status.

A final area where bias is introduced is in jury deliberations. Deliberations ostensibly facilitate the focusing on facts by jurors, rather than assumptions. Focusing on only facts allows individual perceptions to be scrutinized by the whole group. While this is generally accurate, two major research areas have produced evidence undermining the belief that groups can be more rational than individuals.

Classical psychological research indicates some group decisions may actually be more likely to lead to poor decision performance and extreme positions. Other research highlights that group deliberations appear to exaggerate introduced “source monitoring errors” (i.e., misattributed negative pre-trial publicity) and intensify the effects of biasing information.

The Report concludes that “bias is a multifaceted phenomenon introduced from many different elements, and that several sources of bias may interact with one another” throughout a trial. Several remedial suggestions are offered by the authors.

First, utilize jury selection procedures premised on instruments like JBS, JPAQ, and RLAQ to attenuate bias. This helps create “heterogenous juries that challenge problematic biases from individual jurors.”

Second, increase the quality of expert testimony by implementing the following: training designed to assist experts in the communication of their testimony to ensure it is clear, logical, and presented in a way laypersons understand; ensure the independence of experts by disassociating them from either side of the adversarial process; and improve the codes of conduct for expert witnesses.

Third, Linear Sequential Unmasking (“LSU”) should be utilized by experts to diminish sources of bias in their work product. LSU is a technique “that requires examiners not only to first examine the trace evidence in isolation from the reference material, but also provides a balanced restriction on the changes that are permitted post exposure to the reference material.” As an example, the expert should analyze the evidence, e.g., fingerprints, independent of any prior knowledge as to who the perpetrator is suspected of being. LSU “ensures sequencing of the relevant contextual information so that the more objective and less biasing information is [prioritized].”

Fourth, knowledge of the effect that biases have on decision-making processes should be formatted to educate legal professionals and layperson jurors. This may allow jurors to examine any influence that biases might have had on the interpretation of events by a witness.

Finally, continue conducting research on the sources and effects of juror bias in witness testimony, evidence evaluation and interpretation, and juror decision-making generally. This is a significant issue that can only lead to miscarriages of justice if not restrained. By continuing to examine and understand bias in juror decision-making, proper procedures may be established to remove as much of the negative effect of bias as possible. 


As a digital subscriber to Criminal Legal News, you can access full text and downloads for this and other premium content.

Subscribe today

Already a subscriber? Login



Prisoner Education Guide side
Advertise Here 4th Ad
PLN Subscribe Now Ad